Beam centering device for cathode-ray tubes



July 4, 1950 R B. GETHMANN BEAM CENTERING DEVICE FOR CATHODE-RAY TUBES Filed May 21. 1948 Fig.6.

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Patented July 4, 1950 BEAM CENTERING DEVICE FOR CATHODE-RAY TUBES Richard B. Gethmann, Bridgeport, Conn, assignor to General Electric Company, a corporation of New York Application May 21, 1948, Serial No. 28,315

3 Claims. 1

My invention relates to cathode ray tubes of the type which are used in television receivers and has for its object to provide a beam centering device which is simple in adjustment and construction.

In cathode ray tubes which are used in television receivers, it is customary to employ a device for focussing the cathode ray beam and an additional device for deflecting the beam through transverse paths for scanning fluorescent end walls provided in such tubes. In order that a televised scene being reproduced on the end wall of a cathode ray tube be accurately centered on that end wall and that distortion. of the picture be minimized, it is desirable that the cathode ray beam itself be accurately centered relative to the axis of the tube. Where the centering force is applied to the beam prior to: the focussing force, it has been found that a distortion is produced around the edge of the reproduced screen which is known as neck shadow. Consequently, it has been found best to effect centering of the cathode ray beam subsequent to the application. of focus.- singforces. Attempts have been made to effect centering of the beam at the time that deflection forces are applied to the beam, such centering action being obtained either by introducing the unidirectional component to the current supplied to the coils of the deflection yoke, or by mechanical adjustment of the position. of the beam focussing device. It is an object of my invention to. provide a new and improved beam centering device which applies beam centering forces to an electron beam subsequent to the application of fccussing forces thereto and prior to the application of deflecting or scanning forces.

It is still another object of my invention to provide a new and improved beam centering device which employs a plurality of magnetic fields of uniform cross section and which are adjustable relative to each other to effect accurate centering of the electron beam.

One of the features of my invention consists in employing two permanent magnets having high directional characteristics and; adapted to provide a magnetic field of uniform cross section across a narrow portion of the neck of the cathode ray tube. One of the magnets is adjustable in position relative to a magnetic shunt to control the strength and direction of the centering force.

The features of my invention which are believed to be novel and patentable will be pointed out in the claims appended thereto. For a better understanding or my invention reference is made to: the following description and the accompany- 2 ing drawings in which Fig. 1 illustrates a cathode ray tube suitable for use in a television receiver and employing my invention; Fig. 2 is an enlarged view, partly in section, of my improved beam centering device; Fig. 3 is a view of one of the components of the device of Fig. 2; Fig. 4 is a side elevational view, partly in section, of a modification of my beam centering device; Figs. 5 and 6 are views illustrating certain components of. the device of Fig. 4; and Figs. 7 and 8 are views illuse trating additional modifications of my invention.

Referring now to Fig. 1, I have there illustrated a conventional cathode ray tube 1 suitable for use in a television receiver and comprising an evacuated envelope having a neck portion 2 and a fluorescent screen or end wall 3. The cathode ray tube 1 includes a cathode 4, a control electrode 5 and an accelerating electrode 6. Surrounding the neck portion 2 is a piece of tubing l formed of any suitable material which is mechanically secured to a deflection yoke 5 and upon which is positioned a focus coil 8. The focus coil 8 may comprise either an electromagnet or a combined structure including both a permanent magnet and an electromagnet, the current through which may be adjusted to control the focussing effect upon the cathode ray beam of the tube. The deflection yoke 9 may consist of a plurality of coils (not shown) for deflecting the cathode ray beam in both horizontal and vertical directions to scan the fluorescent screen 3 in the usual manner. Positioned between the focus coil 8 and the deflection yoke 2 is a beam centering device comprising a non-magnetic sleeve l 0 having a. longitudinal slot I l therein. Supported on the sleeve Ill are a pair of ring-shaped permanent magnets l2, I3.

The construction of the beam centering device may be more clearly seen in the enlarged view of Fig. 2. The focus coil 8 includes an end plate l4 formed of steel or any other suitable ferromagnetic material having a neck i5 which engages the outer surface of the tube l. Positioned between the sleeve Hiand the tubing 1 is a spacer 16 which supports sleeve ill. One end of sleeve It! is arranged to slide over neck iii of the end plate of the focus coil. The end of sleeve l0 remote from end plate M has a groove H which receives the peripherally incomplete permanent magnet 13. The adjacent ends of the magnet I 3 are bentinwardly to engage slot H inthe end of sleeve l0. Permanent magnet 12, for reasons to be pointed out later, is larger than magnet i3 and likewise has inwardly turned ends which flt into slot H.

Magnets l2, l3 preferably are formed of any suitable permanent magnet material having a magnetic materials of the copper-nickel-iron alloy type, known as cunife, are well adapted for 7 use as the rings l2, it, other magnetic materials, such as the copper-nickel-cobalt alloys including cunico, and the silver-manganese-aluminum alloys including silmanal, are also well adapted for use in this construction. Similarly,

d rigidly secured within the inner surface of sleeve I8 is a second ring-shaped permanent magnet 22. Magnet 22, which may be a solid ring or may be split as illustrated, is located at the edge of sleeve I8 nearer the deflecting yoke. The sleeve i8 is likewise provided with a pair of laterally extending; tabs or ears 23 which assist in rotating the sleeve and its supported magnets relative to the neck l5 and the neck portion of the cathode "ray tube.

- A centering .device constructed as shown in Fig. 4-provides increased control of the field for centering a cathode ray beam in that the magnet 2| is removable and may be rotated through certain sintered type of permanent magnet materials may be employed. Fig. 3 also illustrates the manner in which the free ends of rings 12 and i3 are bent inwardly so that they fit within the slot H. The ring 13 preferably is located in groove El" and is cemented therein. The groove I'I preferably is located as close to the deflection yoke 9 as is practical. In using the beam centering device, the assembly of ringsl2, l3 and the sleeve 50 is free to rotate about the neck portion '2 of the cathode ray tube and likewise about the neck l5 of the end ring M. The neck l5, which is formed of iron or a suitable ferromagnetic material, acts as a shunt for the magnet 12 when the magnet is located in the position shown in Fig. 2. Since, however, a shunt such as the neck i5 is never a perfect shunt for all the magnetic flux, the magnet I3 is provided and is so magnetized that it provides a flux which is adjusted both in field strength and direction to produce a deflection of the cathode ray beam in adirection opposite to that produced by the shunted permanent magnet I2. Moreover, the strength of ring 13 is sufilcient in magnitude approximately to balance out the deflection produced by magnet l2 when shunted by neck 15.

If ring i2 is moved axially of the cathode ray tube in the direction of ring E3, the shunting efiect of neck i5 is lessened in such a anner that, when ring i2 is adjacent ring 53, the electron beam is deflected a predetermined amount from the'tube axis, for example one inch from the tube axis. Thus, by adjustment of the axial position of ring l2 and by rotation of the assembly including rings l2, l3 and sleeve Ill, the center of the television picture may be moved to any point within a one-inch circle. This range has been found suiiicient to give the centering range required in television receivers.

In Fig. 4, I have shown a modification of my beam centering device which is particularly useful for centering the cathode ray beam of a tube in which there is a very small amount of space available between the focus coil 8 and the deflection yoke 9 for locating the beam centering device. In such a case, it is impossible to obtain thedegree of control of beam centering which is possible with the arrangement of Fig. 2 in which the magnet l2 may be moved a considerable distance axially of the magnetic ring l3. In the arrangement of Fig. 4, there is provided a metallic non-magnetic sleeve 18 which is adapted to slip over the neck [5 at the end of the end ring Mof the focus coil. The metallic sleeve I8 is provided on its periphery with a pair of diametrically positioned outwardly protruding portions l9. Adjustably supported for axial movement along the outer surface of the sleeve I3 is a first ring-shaped permanent magnet 2| and an angle ofv to a position in which the magnetic flux thereof is aiding the flux established by magnet 22. In the use of the device of Fig. 4, the neck l5 functions as a magnetic shunt for the permanent magnet 2!. The degree of shunting action obtained depends upon the extent to which the sleeve [8 telescopes neck 15 and the position. of magnet 2i relative to the right-hand edge of sleeve l8. By adjusting the position of sleeve Iii-relative to neck it, more or less shunting of the magnetic flux magnet 2! is obtained. Byadjusting the position of-magnet 2| relative to magnet '22, the strength. of the magnetic field within the neck of the cathode ray tube is controlled. Finally, by rotating the entire assembly comprising sleeve l8 andmagnets 2|, 22, the angular position of the cathode ray beam may be varied. i

Fig. 5' is a side elevational view of permanent magnet 2| and illustrates the uniform magnetic fieldestablished in one direction across the cathode ray tube neck by that magnet. Fig.- 6 is-a side elevational view'of the ring-shaped magnet 22. This'magnet preferably is secured within the end of inner surface of sleeve It by cementing or soldering thereto. The magnetic flux of magnet 22 normally is of a polarity opposite to that established'inthe neck of the cathode ray tube by magnet 2l. However, as explained previously, the free ends of magnet 21 may be spread apart slightly and the magnet rotated through 180 to establish a magnetic field which aids that established by magnet 22. In Fig. '7 I have shown another modiflcation'of my inventionin which two permanent magnet centering rings are mounted so that they may be rotated one'with respect to the other. In the centering ring shown in this figure, one of the rings maybe fixed on a supporting sleeve and. the other ring may be adjustable axially of the I supporting sleeve'and may also be rotatable on the sleeve. Thus, as illustrated, the permanent magnet ring 24 may be fixed on the non-magnetic supporting sleeve 25,-while the permanent magnet ring 26 is'movable axially of sleeve 25 and is rotatable on that sleeve. By axial movement of ring 25, the shunting effected by the ferromagnetic end ring may be controlled. In this construction, the deflecting field is the vector sum of the two fields established by rings 24, 26. When these fields'areopposing and equal in magnitude, no deflection of the electron beam results in the cathode ray tube whose neck is encircled by this structure; When, however, the fields of rings 24, 26 are aiding, a maximum deflection of the cathode ray beam is obtained. By rotating the two rings as'a unitby rotation of the sleeve 25, control of the direction of the resultant deflecting force is obtained.

In Fig. 3, I have shown a still further modification in'which a first permanent magnet ring 21- is supported upon and preferably fixed, a by cementing, to a non-magnetic supporting sleeve 28, while the second permanent magnet ring 29 is arranged concentric and coplanar with ring 27. The ring 29 is provided with tabs 30 by means of which it may be rotated relative to ring 21. A non-magnetic sleeve 31 may be positioned between rings 21, 29 if desired. In this construction, when the rings 27, 29 have the same field strength, a non-magnetic shunt, such as the shunt l5 illustrated in connection with the embodiment of Fig. 7, is not required. In using a beam centering device of the type illustrated in Fig. 8, the deflecting field is the resultant o vector sum of the fields established by rings 27, 29. This is a maximum when the fields are aligned and is zero when the fields are opposed. Preferably, the whole assembly is rotatable relative to the neck of a cathode ray tube by rotation of sleeve 28. Accordingly, by rotation of ring 29 relative to ring 27, the deflecting field may be fixed both in strength and in direction.

An important advantage of my improved beam centering device is that it may be permanently adjusted to provide optimum centering of the cathode ray beam of a particular cathode ray tube. Moreover, the centering effect established is permanent in character and eliminates the necessity of using either electrical centering controls which vary the unidirectional current in the coils of a deflection yoke, or of providing a complex mechanical system for adjusting the focus coil relative to the neck of a cathode ray tube. It is well known that mechanical devices which are used for adjusting the position of a focus coil relative to the neck of a cathode ray tube usually introduce distortion effects. These effects are particularly noticeable and objectionable if a composite focus coil is used in which focussing effects are obtained by both a permanent magnet structure and an electromagnet.

While, in the foregoing, I have described my inproved beam centering device a being applied to a cathode ray tube of the type which employs electromagnetic focussing and deflection means, it is apparent that the invention is applicable equally as well to cathode ray tubes which use electrostatic focussing and electrostatic deflecting means. Accordingly, it will be understood that I do not wish to be limited to the particular embodiment shown, since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A beam centering device for a cathode ray tube of the type having a neck portion through which the cathode ray beam is projected comprising a supporting sleeve, a pair of circular permanent magnets supported on said sleeve, each of said magnets being of the type producing a uniform magnetic field parallel with a diameter thereof, said fields produced by said magnets being parallel, one of said magnets being fixed relative to said sleeve, a circular magnetic shunt slidable within said sleeve, the other of said magnets being movable axially of said sleeve to vary the position thereof relative to said first magnet and said shunt.

2. A beam centering device for a cathode ray tube of the type having a neck portion and means for projecting a cathode ray beam axially of said neck portion comprising, a non-magnetic supporting sleeve adapted to encircle the neck of a cathode ray tube, a pair of peripherally incomplete ring-shaped permanent magnets supported on said sleeve, said magnets producing parallel uniform magnetic fields across said sleeve in a direction normal to the axis thereof, the field of one of said magnets having a polarity opposite to that of the other, a first of said magnets being attached to said sleeve in a predetermined position axially thereof, the other of said magnets being movable axially of said sleeve, said sleeve and other magnet having portions resisting rotation of said other magnet relative to said sleeve, and magnetic means extending within said sleeve for shunting a variable amount of the flux of said other magnet as the position of said other magnet is varied axially of said sleeve,

3. A beam centering device for a cathode ray tube of the type having a neck portion through which the cathode ray beam is projected comprising a supporting sleeve, a pair of circular permanent magnets supported on said sleeve, each of said magnets being of the type producing a uniform magnetic field parallel with a diameter thereof, one of said magnets being fixed relative to said sleeve, a circular magnetic shunt slidable within said sleeve, the other of said magnets being movable axially of said sleeve to vary the position thereof relative to said first magnet and said shunt.

RICHARD B. GETHMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,102,421 Kuehni Dec. 14, 1937 2,200,039 Nicoll May 7, 1940 2,224,933 Schlesinger Dec. 17, 1940 2,418,487 Sproul Apr. 8, 1947 2,431,077 Poch Nov. 18, 1947 2,442,975 Grundmann June 8, 1948 2,456,474 Wainwright Dec. 14, 1948 2,460,609 Torsch Feb. 1, 1949 FOREIGN PATENTS Number Country Date 464,637 Great Britain Apr. 21, 1937 

